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Pickering emulsions and foams as well as capillary suspensions are becoming increasingly more popular as inks for 3D printing. However, a lack of understanding of the bulk rheological properties needed for their application in 3D printing is potentially stifling growth in the area, hence the timeliness of this review. Herein, we review the stability and bulk rheology of these materials as well as the applications of their 3D-printed products. By highlighting how the bulk rheology is tuned, and specifically the inks storage modulus, yield stress and critical balance between the two, we present a rheological performance map showing regions where good prints and slumps are observed thus providing clear guidance for future ink formulations. To further advance this field, we also suggest standard experimental protocols for characterizing the bulk rheology of the three types of ink: capillary suspension, Pickering emulsion and Pickering foam for 3D printing by direct ink writing.
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Achieving the delicate balance required for both emulsion and gel characteristics, while also imparting biological functionality in gelled emulsions, poses a significant challenge. Herein, Pickering emulsion biogels stabilized is reported by novel biological nanofibrils assembled from natural glutathione (GSH) and a tripod cholic acid derivative (TCA) via electrostatic interactions. GSH, composed of tripeptides with carboxyl groups, facilitates the protonation and dissolution of TCA compounds in water and the electrostatic interactions between GSH and TCA trigger nanofibrillar assembly. Fibrous nuclei initially emerge, and the formed mature nanofibrils can generate a stable hydrogel at a low solid concentration. These nanofibrils exhibit efficient emulsifying capability, enabling the preparation of stable Pickering oil-in-water (O/W) emulsion gels with adjustable phase volume ratios. The entangled nanofibrils adsorbed at the oil-water interface restrict droplet movement, imparting viscoelasticity and injectability to the emulsions. Remarkably, the biocompatible nanofibrils and stabilized emulsion gels demonstrate promising scavenging properties against reactive oxygen species (ROS). This strategy may open new scenarios for the design of advanced emulsion gel materials using natural precursors and affordable building blocks for biomedical applications.
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HYPOTHESIS: Organohydrogel emulsions display unique rheological properties and contain hydrophilic and lipophilic domains highly desirable for the loading of active compounds. They find utility in various applications from food to pharmaceuticals and cosmetic products. The current systems have limited applications due to complex expensive formulation and/or processing difficulties in scale-up. To solve these issues, a simple emulsification process coupled with unique compounds are required. EXPERIMENTS: Here, we report an organohydrogel emulsion based only on a low concentration of 12-hydroxystearic acid acting as a gelling agent for both oil and water phases but also as a surfactant. The emulsification process is based on in-situ surfactant transfer. We characterize the emulsification process occurring at the nanoscale by using tensiometry experiments. The emulsion structure was determined by coupling Small Angle X-ray and neutron scattering, and confocal Raman microscopy. FINDINGS: We demonstrate that the stability and unique rheological properties of these emulsions come from the presence of self-assembled crystalline structures of 12-hydroxystearic acid in both liquid phases. The emulsion properties can be tuned by varying the emulsion composition over a wide range. These gelled emulsions are prepared using a low energy method offering easy scale-up at an industrial level.
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Our understanding of the microstructure of many plant proteins is based on the ancient and conventional methods of alkali extraction and acid precipitation, which generate considerable amounts of NaCl causing salting-out effects and aggregation of their molecules. In this study, monodisperse rice protein (RP) nanoparticles were prepared using cation-exchange resins that release H+ and absorb Na+, thus avoiding the generation of NaCl during neutralization of the alkali extracts. The generated RP nanoparticles of small diameter (20 nm) and excellent uniformity (0.17 polydispersity) quickly diffuse to and stabilize the oil-water interface, producing oil-in-water Pickering emulsions. The emulsifying ability and emulsion stability afforded with these nanoparticles were 17 and 3.5 times higher than those of nanoparticles prepared by conventional alkali extraction and acid precipitation methods, respectively. Furthermore, increased RP nanoparticle concentration created more stable emulsions with smaller droplets and reduced flocculation index vital for practical applications. This study provides a convincing example of how to prepare monodisperse protein nanoparticles that adsorb at a fluid interface, which may find numerous applications in food and cosmetic formulations.
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The impact of liquid marbles coated with a diversity of hydrophobic powders with various solid substrates, including hydrophobic, hydrophilic, and superhydrophobic ones, was investigated. The contact time of the bouncing marbles was studied. Universal scaling behavior of the contact time tc as a function of the Weber number (We) was established; the scaling law tc = tc(We) was independent of the kind of powder and the type of solid substrate. The total contact time consists of spreading time and retraction time. It is weakly dependent on We and this is true for all kinds of studied powders and substrates. This observation hints to the surface tension/inertia spring model governing the impact. By contrast, the spreading time ts scales as [Formula: see text], n = 0.28 - 0.30 ± 0.002. We relate the origin of this scaling law to the viscous dissipation occurring within the spreading marbles. The retraction time tr grows weakly with the Weber number. The scaling law was changed at threshold values of We â 15-20. It is reasonable to explain this change with the breaking of the Leidenfrost regime of spreading under high values of We.
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Retaining emulsions stable at high acidity and salinity is still a great challenge. Here, we report a novel multi-headgroup surfactant (C3 H7 -NH+ (C10 COOH)2 , di-UAPAc) which can be reversibly transformed among cationic, anionic and zwitterionic forms upon pH variation. Stable oil-in-dispersion (OID) emulsions in strong acidity (pH=2) can be co-stabilized by low concentrations of di-UAPAc and silica nanoparticles. High salinity at pH=2 improves the adsorption of di-UAPAc on silica particles through hydrogen bonding, resulting in the transformation of OID emulsions into Pickering emulsions. Moreover, emulsification/demulsification and interconversion between OID and Pickering emulsions together with control of the viscosity and droplet size can be triggered by pH. The present work provides a new protocol for designing surfactants for various applications in harsh aqueous media, such as strong acidity and high salinity, involved in oil recovery and sewerage treatments.
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The past decades have witnessed the development of various stimuli-responsive materials with tailored functionalities, enabling droplet manipulation through external force fields. Among different strategies, light exhibits excellent flexibility for contactless control of droplets, particularly in three-dimensional space. Here, we present a facile synthesis of plasmonic hybrid microgels based on the electrostatic heterocoagulation between cationic microgels and anionic Au nanoparticles. The hybrid microgels are effective stabilizers of oil-in-water Pickering emulsions. In addition, the laser irradiation on Au nanoparticles creats a "cascade effect" to thermally responsive microgels, which triggers a change in microgel wettability, resulting in microgel desorption and emulsion destabilization. More importantly, the localized heating generated by a focused laser induces the generation of a vapor bubble inside oil droplets, leading to the formation of a novel air-in-oil-in-water (A/O/W) emulsion. These A/O/W droplets are able to mimic natural microswimmers in an aqueous environment by tracking the motion of a laser spot, thus achieving on-demand droplet merging and chemical communication between isolated droplets. Such proposed systems are expected to extend the applications of microgel-stabilized Pickering emulsions for substance transport, programmed release and controlled catalytic reactions.
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HYPOTHESIS: Biocatalysts are key to the realization of all bioconversions in nature. However, the difficulty of combining the biocatalyst and other chemicals in one system limits their application in artificial reaction systems. Although some effort, such as Pickering interfacial catalysis and enzyme-immobilized microchannel reactors, have addressed this challenge an effective method to combine chemical substrates and biocatalysts in a highly efficient and re-usable monolith system is still to be developed. EXPERIMENTS: A repeated batch-type biphasic interfacial biocatalysis microreactor was developed using enzyme-loaded polymersomes in the void surface of porous monoliths. Polymersomes, loaded with Candida antarctica Lipase B (CALB), are fabricated by self-assembly of the copolymer PEO-b-P(St-co-TMI) and used to stabilize oil-in-water (o/w) Pickering emulsions as a template to prepare monoliths. By adding monomer and Tween 85 to the continuous phase, controllable open-cell monoliths are prepared to inlay CALB-loaded polymersomes in the pore walls. FINDINGS: The microreactor is proven to be highly effective and recyclable when a substrate flows through it, which offers superior benefits of absolute separation to a pure product and no enzyme loss. The relative enzyme activity is constantly maintained above 93% in 15 cycles. The enzyme is constantly present in the microenvironment of the PBS buffer ensuring its immunity to inactivation and facilitating its recycling.
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Reactores Biológicos , Catálisis , BiocatálisisRESUMEN
Poly(ε-caprolactone) (PCL)-incorporated lignin-chitosan biomass-based nanocomposite porous scaffolds have been effectively prepared by templating oil-in-water Pickering high internal phase emulsions (HIPEs). PCL is dissolved in oil and chitosan and lignin nanoparticles originate in water. The continuous phase of the emulsions is gelled by cross-linking of chitosan with genipin and then freeze-dried to obtain porous scaffolds. The resulting scaffolds display interconnected and tunable pore structures. An increase in PCL content increases the mechanical strength and greatly reduces the water absorption capacity of the scaffolds. Scaffolds loaded with the anti-bacterial drug enrofloxacin show a slow drug release profile, adjustable release rate, and favorable long-term anti-bacterial activity. Moreover, Pickering emulsion templates with suitable viscosity are used as 3D printing inks to construct porous scaffolds with personalized geometry. The results imply that the simplicity and versatility of the technique of combining freeze-drying with Pickering HIPE templates is a promising approach to fabricate hydrophobic biopolymer-incorporated biomass-based nanocomposite porous scaffolds for biomedical applications.
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Construction of well-ordered two-dimensional (2D) and three-dimensional (3D) assemblies using one-dimensional (1D) units is a hallmark of many biointerfaces such as skin. Mimicking the art of difunctional properties of biointerfaces, which skin exhibits as defense and shelter materials, has inspired the development of smart and responsive biomimetic interfaces. However, programming the long-range ordering of 1D base materials toward vigorous control over 2D and 3D hierarchical structures and material properties remains a daunting challenge. In this study, we put forward construction of 3D enteric biomaterials with a two-strata 2D Janus interface assembled from self-adaptation of 1D protein-polysaccharide nanostructures at an oil-water interface. The biomaterials feature a protein dermis accommodating oil droplets as a reservoir for bioactive compounds and a polysaccharide epidermis protecting them from gastric degradation. Furthermore, the epidermis can be fine-tuned with different thicknesses rendering enteric delivery of a bioactive cargo (coumarin-6) with controllable retention in the intestinal tract from 6 to 24 h. The results highlight a skin-inspired construction of enteric biomaterials by self-adaptation of 1D nanostructures at the oil-water interface toward 2D Janus biointerfaces and 3D microdevices, which can be tailored for intestinal treatments with intentional therapeutic efficacies.
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Materiales Biocompatibles , Nanoestructuras , Nanoestructuras/química , AguaRESUMEN
A concept of interfacial competitive reaction between biomineralization and alginate gelation at an all-aqueous single-emulsion droplet interface to prepare robust coconut-like capsules (inner hard wall and outer soft wall) is developed. The concept is further applied for enzyme immobilization with high encapsulation efficiency, enzyme loading, mass transfer coefficient, and recyclability. The thickness and swelling properties of the shell are simply tunable by a competitive reaction. Our platform may open a green, facile, and efficient way to prepare organic-inorganic hybrid sustainable materials with tailored compositions and structures.
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Cocos , Enzimas Inmovilizadas , Cápsulas/química , Enzimas Inmovilizadas/químicaRESUMEN
A novel nonionic-anionic Bola surfactant (abbreviated as CH3O(EO)7-R11-COOH) was designed and synthesized by condensation of methyl polyoxyethylene (7) ether with 12-bromododecanoic acid. In neutral aqueous solution, the surfactant behaves as a nonionic one and can stabilize oil-in-water (O/W) conventional emulsions alone and costabilize O/W Pickering emulsions with positively charged alumina nanoparticles with n-decane as the oil. In alkaline solution, the carboxylic acid group is deprotonated, becoming anionic and the surfactant is converted to Bola form, which is an inferior emulsifier and does not adsorb on particle surfaces, resulting in demulsification of both kinds of emulsions. With strong hydrophilicity, both the Bola surfactant and the bare particles return to the aqueous phase after demulsification, which is therefore recyclable and reusable in accordance with sustainable chemistry and engineering. In acidic media between pH 3 and 6, the ethyleneoxy groups tend to desorb from particle surfaces, slightly reducing the hydrophobicity of the particles. However, Pickering emulsions are still stable but their droplet size increases on lowering the pH. The Pickering emulsions are therefore pH-responsive and size-controllable. This newly designed Bola surfactant is effective in preparing smart emulsions, which are extensively applied in heterogeneous catalysis, oil product transportation, emulsion polymerization, and new material preparation.
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Phase inversion of Pickering emulsions from water-in-oil (W/O) to oil-in-water (O/W) is achieved by the formation of an interfacial particle bilayer using negatively charged and positively charged particles dispersed in water and oil, respectively, before emulsification. A mechanism based on electrostatic attraction across the toluene-water interface is proposed and verified by systematic investigation of the parameters that affect the surface charge of negatively charged particles such as pH and salt concentration. Cationic silica-FITC particles (600 nm) can be dispersed in toluene and stabilize W/O emulsions alone; phase inversion of this emulsion can be induced by the addition of anionic silica-RB particles in the aqueous phase at a concentration of 1.0 wt % or above. It is revealed that silica-RB particles of a smaller size (100 nm) can induce emulsion phase inversion at a much lower concentration (0.4 wt %) and an interfacial particle bilayer is clearly revealed by CLSM and SEM images. By tuning the surface charge density of silica-RB particles, the electrostatic attraction mechanism leading to the formation of the interfacial particle bilayer is confirmed and emulsion stability can be tuned as demonstrated by osmotic pressure enhancement results obtained from centrifugation.
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A ferrocene surfactant can be switched between single and double head form (FcN+ C12 /Fc+ N+ C12 ) triggered by redox reaction. FcN+ C12 can neither stabilize an O/W emulsion alone nor an oil-in-dispersion emulsion in combination with alumina nanoparticles due to the steric hindrance of the ferrocene group. However, such steric hindrance can be overcome by increasing the charge density in Fc+ N+ C12 , so that oil-in-dispersion emulsions can be co-stabilized by Fc+ N+ C12 and alumina nanoparticles at very low concentrations (1×10-7 â M (≈50â ppb) and 0.001â wt %, respectively). Not only can reversible formation/destabilization of oil-in-dispersion emulsions be achieved by redox reaction, but also reversible transformation between oil-in-dispersion emulsions and Pickering emulsions can be obtained through reversing the charge of alumina particles by adjusting the pH. The results provide a new protocol for the design of surfactants for stabilization of smart oil-in-dispersion emulsions.
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Sorbitan esters have been extensively used as surfactants to stabilize emulsions in many fields. However, the preparation of an oleofoam with sorbitan ester alone has not been reported. Here, we apply a novel protocol to fabricate stable oleofoams of high air volume fraction from mixtures of vegetable oil and sorbitan ester. To incorporate more air bubbles into the oil matrix, aeration is first carried out in the one-phase region at high temperatures, during which the highest over-run can reach 280%. Due to foam instability at high temperatures, the foam is then submitted to rapid cooling, followed by storage at low temperatures. For high-melting sorbitan monostearate, the resulting foams containing many crystal-encased air bubbles are ultrastable to drainage, coarsening, and coalescence for several months. On the contrary, the cooled foams with low-melting sorbitan monooleate go through a gradual decay lasting for more than 1 month. We highlight the importance of hydrogen bond formation between surfactant and oil in enhancing foam stability. The generic nature of the above findings is demonstrated by preparing oil foams with various vegetable oils and sorbitan monooleate.
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The rising level of obesity is often attributed to high sugar and/or fat consumption. Therefore, the food industry is constantly searching for ways to reduce or eliminate sugar or fat in food products. Therefore, honey foam, which contains little sugar and no fat, can be used as cake, cracker or bread spread instead of butter or margarine which contains a substantial amount of fat or jam that contains a substantial amount of sugar. Small solid particles (nanometers to micrometers) of suitable wettability are now considered outstanding foam-stabilizing agents. However, while the degree of particle wettability necessary to obtain very stable aqueous and nonaqueous foams is well-known, that needed to obtain very stable honey foam is unknown. In this study, the influence of the degree of wettability of fumed silica particles, indicated by their % SiOH (14-100), was investigated in honey in relation to foam formation and foam stability. The honephilic particles (61%-100% SiOH) formed particle dispersion in honey, while foams were obtained with the honephobic particles (14%-50% SiOH). The thread-off between particle dispersion and foam formation occurs at 50% SiOH, meaning foam formation in honey is possible when the particles are at least 50% honephobic. At relatively low particle concentration <1 wt.%, foam volume decreases with increasing honephobicity, but increases with honephobicity at relatively high concentration >1 wt.%. Also, as particle concentration increases, the shape of the air bubbles in the foam changes from spherical to non-spherical. After a little drainage, the foams remain stable to drainage and did not coalesce substantially for more than six months. These findings will guide the formulation of edible Pickering honey foams.
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Miel , Mantequilla , Excipientes , Margarina , Dióxido de Silicio , Azúcares , HumectabilidadRESUMEN
The study of liquid marbles (LMs) composed of stabilizing liquid droplets with solid particles in a gaseous environment has matured into an established area in surface and colloid science. The minimized "solid-liquid-air" triphase interface enables LMs to drastically reduce adhesion to a solid substrate, making them unique non-wetting droplets transportable with limited energy. The small volume, enclosed environment, and simple preparation render them suitable microreactors in industrial applications and processes such as cell culture, material synthesis, and blood coagulation. Extensive application contexts request precise and highly efficient manipulations of these non-wetting droplets. Many external fields, including magnetic, acoustic, photothermal, and pH, have emerged to prepare, deform, actuate, coalesce, mix, and disrupt these non-wetting droplets. Electric fields are rising among these external stimuli as an efficient source for manipulating the LMs with high controllability and a significant ability to contribute further to proposed applications. This Feature Article attempts to outline the recent developments related to LMs with the aid of electric fields. The effects of electric fields on the preparation and manipulation of LMs with intricate interfacial processes are discussed in detail. We highlight a wealth of novel electric field-involved LM-based applications and beyond while also envisaging the challenges, opportunities, and new directions for future development in this emerging research area.
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HYPOTHESIS: Oil-in-dispersion emulsions can be stabilized by like charged particles and surfactant. Surfactant adsorbs at the oil-water interface to reduce the interfacial tension and endow the interface with charge, while particles remain dispersed in the aqueous phase to provide electrostatic repulsion between droplets and particles. Can weakly surface-active organic electrolytes adsorb at the oil-water interface and behave like surfactants in stabilizing oil-in-dispersion emulsions with like charged particles? EXPERIMENTS: Symmetrical organic electrolytes, tetraalkylammonium bromides (R4NBr), with either no or very low interfacial activity endowing oil droplets with charge were combined with alumina nanoparticles to stabilize emulsions. The effect of R chain length (varying from methyl to butyl) on the type and stability of emulsions was investigated. FINDINGS: Mixtures of high concentrations of short chain R4NBr salts (R = methyl or ethyl) and alumina particles stabilise oil-in-water Pickering emulsions, whereas longer chain (R = propyl or butyl) analogues stabilize oil-in-dispersion emulsions assisted by alumina particles. Tetrapropylammonium and tetrabutylammonium cations adsorb at the oil-water interface reducing the interfacial tension and endowing the interface with charge. The stability of the oil-in-dispersion emulsions is dominated by the electrostatic repulsion between the droplets and between droplets and particles in the continuous aqueous phase.
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HYPOTHESIS: Liquid marbles i.e. droplets coated by hydrophobic particles may be formed not only on the solid substrates but also on the floating layers of hydrophobic powders such as fluorinated fumed silica or polytetrafluoroethylene. EXPERIMENTS: Formation and growth of liquid marbles on fluorinated fumed silica or polytetrafluoroethylene powder floating on a heated water-vapor interface is reported. Marbles emerge from condensation of water droplets levitating above the powder layer. FINDINGS: The kinetics of the growth of droplets is reported. Growth of droplets results from three main mechanisms: water condensation, absorption of small droplets and merging of droplets with neighboring ones. Growing droplets are coated with the hydrophobic powder, eventually giving rise to the formation of stable liquid marbles. Formation of hierarchical liquid marbles is reported. Growth of liquid marbles emerging from water condensation follows the linear temporal dependence. A phenomenological model of the liquid marble growth is suggested.
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HYPOTHESIS: Alcohols can strongly reduce the interfacial tension between immiscible liquids, thus facilitating the formation of emulsions. By combining non-surface-active hydrophobic particles with medium-chain alcohols, stable water-in-oil (w/o) high internal phase Pickering emulsions (HIPPEs) can be easily prepared without high-energy emulsification methods. EXPERIMENTS: The emulsions containing acrylate monomer as the oil phase were prepared at different pH values in the presence of hydrophobic silica particles. Further, by replacing monomer oil with organic solvents (e.g., toluene) and a certain concentration of alcohol, the promoted particle adsorption at the oil-water interface has been systematically investigated. The morphology and interfacial structure of HIPPEs were visualized by confocal laser scanning microscopy (CLSM). FINDING: At high pH, stable water-in-acrylate monomer HIPPEs can be formed using commercial fumed silica nanoparticles alone with simple stirring or vortexing. The hydrolysis of the acrylate group at high pH can generate alcohols in situ which adsorb at the oil-water interface to reduce the interfacial tension and promote particle adsorption to hinder droplet coalescence. The novel strategy for forming stable and processable HIPPEs can be universally applied to different hydrophobic silica particles with the help of various alcohols as the co-stabilizer, which provides a flexible approach for the fabrication of lightweight, closed-cell solid foams for a range of applications.